1. Field of the Invention
The present invention relates to a plasma torch that is employed in plasma processing such as cutting or welding of work using a plasma arc, and relates to a plasma torch in which the cooling performance of the shield cap that is provided at the front end of the nozzle is enormously improved.
The present invention also relates to improvements in the construction of a retainer cap for holding a nozzle.
Yet further, the present invention relates to improvements in construction for miniaturizing the size of the torch.
2. Description of the Related Art
In general terms, plasma torches are known wherein a nozzle arranged so as to cover an electrode with a plasma gas passage interposed therebetween is provided at the front end of a torch body, a plasma arc being generated between the electrode and a workpiece, through the orifice of the nozzle. A Plasma torch has been proposed (Japanese Patent KOHYO Publication No. H. 6-508793) wherein a plasma jet of high temperature and high speed and of high directionality at high temperature is formed by strongly pinching a plasma arc by forming a long and thin nozzle orifice, in order to raise the quality of cutting of work. In this torch, the heat load of the nozzle is large since the nozzle is narrow and long, and hence direct water cooling is performed by bringing cooling water into contact with the nozzle. A plasma torch has also been proposed wherein a shield cap is provided at the periphery of the nozzle, and an insulator is interposed therebetween. In this torch, not only the cooling of the nozzle as described above, but also cooling of the shield cap is necessary. Conventionally, secondary gas was supplied to the periphery of the plasma arc and the shield cap was cooled by this secondary gas, or the shield cap was cooled by indirect water cooling.
Various types of action of the secondary gas have been proposed, such as for example adjusting the inclination of the cutting surface of a workpiece by applying a rotary flow of this secondary gas (Patent No. 689310). In this case, precise uniformity of the flow of secondary gas is required.
However, in the prior art, with cooling by secondary gas or cooling by indirect water cooling, it is not possible to cool the shield cap sufficiently. If the cooling is insufficient, there us the problem that, when dross flew up during piercing, molten metal at high temperature easily adhered to the shield cap, promoting deterioration of this shield cap. Deterioration of the aperture of the outlet of the secondary gas formed on this shield cap adversely affects uniformity of the secondary gas flow, causing the problem of deterioration of cutting quality of a workpiece.
Also, a plasma torch is known wherein, in order to form the optimum atmosphere surrounding the plasma arc, or in order to raise cooling efficiency of the components peripheral to the nozzle, or in order to control the bevel angle with high precision, secondary gas (assist gas) is supplied to the periphery of the plasma arc. In this conventional plasma torch wherein the secondary gas is supplied, there are provided an inside retainer cap surrounding and retaining the nozzle, a cooling water passage provided at the periphery of the nozzle on the inside of the inside retainer cap, a shield cap covering and protecting the tip of the nozzle, an outside retainer cap surrounding the inside retainer cap and holding the shield cap, and a secondary gas passage formed in the annular space between the inside retainer cap and outside retainer cap. With this arrangement, the task of replacing consumable components such as the nozzle and electrode becomes difficult, since, when replacing the consumable components, a plurality of caps such as the inside retainer cap, shield cap and outside retainer cap must be removed. In order to solve this problem, there has previously been proposed a torch in which the plurality of caps are unified (Laid-open Japanese Patent Application No. 9-285868).
Irrespective of whether or not the plurality of caps were unified, in the usual conventional plasma torch construction, since a secondary gas passage was constituted by the space between the outside retainer cap and inside retainer cap and the cooling water passage was constituted by the space on the inside of the inside retainer cap (space between the inside cap and the nozzle), the outside retainer cap was separated from the cooling water passage by the secondary gas passage, so, as a result, cooling of the outside retainer cap was performed exclusively by the secondary gas. However, during cutting operation, the outside retainer cap was heated by radiation from the plasma arc, but the cooling of the outside retainer cap performed merely by air cooling by the secondary gas was insufficient, with the result that, when replacing the consumable components after completion of the cutting operation, the outside retainer cap could not be touched by operator's hands since it was at high temperature; thus there was the problem that the component replacing operation could not be commenced without waiting for this to cool.
Conventionally therefore a technique was proposed for raising the cooling effect by increasing the flow rate of secondary gas (Japanese Patent KOHYO Publication No. 2-504603). However, assuming that industrial gases such as nitrogen or oxygen were employed as the secondary gas, the problem arose of increased running costs, due to the increased consumption of the secondary gas. A construction has also been proposed (Laid-open Japanese Patent Application No. 5-84579) wherein the outside retainer cap has a hollow wall into which a cooling water is supplied, so that the outside retainer cap is water-cooled. However, even with this construction in which the outside retainer cap was water-cooled, a plurality of caps are still present, and so the operation of replacing the consumable components is troublesome.
Basically, the construction of a plasma torch consists of a coaxial assembly of a plurality of cylindrical components such as electrode, nozzle and shield cap, wherein cooling water passages and various gas passages such as a plasma gas passage and an assist gas passage are provided between those cylindrical components. The size of the torch as a whole is determined by the size of those components, but one of the main factors that govern the torch size is the cooling capacity that is required to cool the torch. The larger the rated arc current of the torch, the greater is the required cooling capacity, and so the larger is the torch size. For example, in the case of a torch of small arc current such as 40A, water cooling is applied to the electrode only, and the other components such as the nozzle and the various caps can be air-cooled by assist gas, and, furthermore, the cooling water passage within the electrode need only to be small. In contrast, in the case of a torch of large arc current such as 300A, the cooling water passages must be wide, water cooling must be applied not only to the electrode but also to the other components such as the nozzle and the caps, and hence all of those components are provided with water-cooling passages, with the result that the construction of the torch is complicated and the torch size is considerably large. Also, in the case of a torch of an intermediate-current such as 120A, an intermediate construction is adopted wherein for example the electrode and nozzle are water-cooled but the caps are air-cooled, and the torch size also is intermediate.
Conventional plasma torches, due to differences in the required cooling capacity depending on the rated arc current value, have different constructions, and the larger the current value, the more complicated does the construction become and the size also becomes larger.
However, decreasing the size of the torch as far as possible is advantageous from various aspects such as ease of the processing operation, ease of torch movement control, and lowering of construction costs and lowering of running costs due to the consumable components such the nozzle and electrode becoming small. There is therefore a demand to further reduce in size in particular large-current torches. Also, it is advantageous if a single type of torch can be generally utilized for various arc current values. For example, if a torch of a size rivaling that of the conventional intermediate current torches could be used not merely for intermediate currents but also for large currents, it would be advantageous in various business aspects such as investment in and maintenance of the plasma processing plant. The greatest problem in meeting this demand is how to ensure high cooling capability in a torch of small size. Specifically, the cooling capability depends on the cooling water flow rate, and so the cooling capability correspondingly increases by making larger the cooling water flow rate. A simple method for increasing the cooling water flow rate is to raise the delivery pressure of the pump for circulating the cooling water. However, the resistance (pressure loss) of the water circuit is proportional to the square of the water flow rate, so, even if the pump discharge pressure is considerably raised, the benefit of a satisfactory increase in water flow rate cannot be expected. Also, from the point of view of torch construction, measures to reduce pressure loss of the water circuit become necessary.
An object of the present invention is therefore to provide a plasma torch capable of sufficiently cooling the shield cap.
A further object of the present invention is to increase the efficiency of the task of replacing consumable components of the plasma torch. Specifically, this object consists in making it possible to commence the component replacing task soon after completion of cutting operation, by eliminating the need to remove a plurality of caps when replacing the consumable components, and by achieving excellent cooling of the outside retainer cap during cutting operation.
Yet a further object of the present invention is to provide a plasma torch wherein the pressure loss of the water circuit is small even though it is of small size, so that a large cooling capacity can be exhibited.